Objectives: Multidrug-resistant Enterobacteriaceae pose a significant threat to public health. We aimed to study the impact of sewage treatment effluent on antibiotic resistance reservoirs in a river. Methods: River sediment samples were taken from downstream and upstream of a waste water treatment plant (WWTP) in 2009 and 2011. Third-generation cephalosporin (3GC)-resistant Enterobacteriaceae were enumerated. PCR-based techniques were used to elucidate mechanisms of resistance, with a new two-step PCR-based assay developed to investigate blaCTX-M-15 mobilization. Conjugation experiments and incompatibility replicon typing were used to investigate plasmid ecology. Results: We report the first examples of blaCTX-M-15 in UK river sediment; the prevalence of blaCTX-M-15 was dramatically increased downstream of the WWTP. Ten novel genetic contexts for this gene were identified, carried in pathogens such as Escherichia coli ST131 as well as indigenous aquatic bacteria such as Aeromonas media. The blaCTX-M-15 gene was readily transferable to other Gram-negative bacteria. We also report the first finding of an imipenem-resistant E. coli in a UK river. Conclusions: The high diversity and host range of novel genetic contexts proves that evolution of novel combinations of resistance genes is occurring at high frequency and has to date been significantly underestimated. We have identified a worrying reservoir of highly resistant enteric bacteria in the environment that poses a threat to human and animal health.